This invention relates to a rotor for an electromechanical machine. More particularly, the invention is directed to the exterior faces of a rotor pole piece which are segmented so as to reduce the eddy currents generated by the rotor.
One type of known electromechanical machine is of a Lundell type rotor comprising a rotor shaft, two iron pole pieces, a fan, slip rings, and a rotor coil. Many Lundell type rotors are used in automobiles to generate the magnetic field within an alternator. The alternator generates electrical power for the various components utilized by the automobile. However, since the alternator is driven by the engine, it has a substantial impact on fuel economy. The average efficiency of a conventional Lundell style alternator is less than 60%. It is therefore critical and strategic to alternator and automobile manufacturers to develop innovative ways to improve alternator efficiency for increasing the fuel economy.
In order to improve the efficiency of any electrical machine it is necessary to reduce the losses within the machines. The losses can generally be categorized as either mechanical, electrical, or core losses. Core losses consist of both eddy current losses and hysteresis losses. Eddy currents are generated from the cyclic magnetization within the iron of the magnetic circuit creating parasitic currents resulting in lost energy dissipated through heat. Eddy currents are generated in both the stator core and the rotor pole finger faces.
The stator generally has the greatest potential to have excessive eddy currents since the rotor is providing each stator tooth with a pulsating north and south magnetic field as it rotates past each tooth. Therefore, the stator is typically laminated to separate the body of the stator core into thin layers that reduce the cross sectional area in the core in which the eddy currents flow. This results in the eddy currents being confined within small areas reducing the net effect on heat generated. This is due to the small areas yielding less variation in flux density within the area than larger areas. The eddy currents are governed by the equation V=IR and V=N(dp/dt) where N=1 in a solid section of iron. In addition, with the known fact that the power loss can be described as P=I{circumflex over ( )}2R it can be seen that smaller areas yield smaller currents resulting in lower losses since the current term is squared in the power loss equation.
The rotor""s field is generally DC and therefore does not produce a varying magnetic field. However, on the surface of the poles the magnetic field is varying as the rotor is rotated due to the permanence of each stator tooth passing by and the emf produced by currents flowing in the stator windings that affect the field from the rotor. This results in a cyclic time varying magnetic field density on the surface of the pole fingers leading to eddy current losses on the pole face. The loss results in excessive heating of the pole face reducing alternator efficiency.
The prior art discloses rotors with Lundell type cores, including rotor pole units with pole claws that have spiral grooves on the outer circumference of the exterior surface of each pole claw. The grooves are formed parallel to one another at equal intervals from the tip of the exterior face of the pole claw to the base of the pole claw in a spiral pattern. The methods for producing the grooves employ conventional finish outer diameter machining. The spiral grooves are partially effective in reducing the eddy currents generated on the pole claw, however, the eddy currents are still allowed to run between the grooves the entire cross-width of the pole face.
Consonant with the present invention, a rotor assembly for an electromechanical machine which reduces the eddy currents generated on the exterior faces of the rotor pole pieces and increases the overall efficiency of the electromechanical machine, which is simple and inexpensive to manufacture, has surprisingly been discovered. The rotor assembly comprises a rotor pole piece having a material composition substantially of iron. A plurality of finger claws are positioned as part the rotor pole piece on the outer circumference of the rotor pole. On the outer surface of each finger claw is an exterior face. Indentations are produced on the surface of the exterior face forming a plurality of closed surface areas. The closed surface areas reduce the eddy currents generated in the rotor assembly for an electromechanical machine thereby increasing the overall efficiency of the machine.